1. Field of the Invention
The invention relates to current mirror circuits which provide output currents that are substantially identical in magnitude and direction to input currents. Such current mirrors are used in linear integrated circuits such as differential amplifiers, biasing circuits, regulators and voltage references to provide biasing and loading, for instance.
2. Prior Art
Bipolar current mirror circuits which are suitable for many applications have been developed in the prior art. For example, one such prior art configuration includes an input PNP transistor and an output PNP transistor having emitter electrodes connected to a positive supply conductor and base electrodes which are connected together. A feedback PNP transistor has a base electrode connected to the collector electrode of the input transistor, an emitter electrode connected to the commonly connected base electrodes of the input and output transistors and a collector electrode connected to a negative reference potential conductor. The collector electrode of the input transistor receives an input current and the collector electrode of the output transistor provides the mirrored output current.
Unfortunately, the magnitudes of the input and output currents of the foregoing mirror circuit are not always substantially equal. More specifically, the input current has a magnitude equal to the sum of the collector current of the input transistor and the base current of the feedback transistor. Since the configuration forces the collector currents of the input and output transistors to be equal, the magnitude of the output current differs from the magnitude of the input current by an error current which is equal to the base current of the feedback transistor. The base current of the feedback transistor is a function of the magnitude of the input current, the beta of the input transistor and the beta of the feedback transistor. The betas of the input and feedback transistors vary with temperature and processing. Also the betas of the PNP transistors made with linear processes tend to be quite low and may vary from one to one hundred depending upon the process, geometry, and operating currents. Thus, the magnitude of the error current is difficult to predict and to cancel. Therefore the foregoing prior art circuit is not suitable for some applications requiring precise duplication of the input current by the output current.
Another problem with the above described prior art circuit is that the input terminal of the current mirror is forced to be at a voltage of at least 2 diode drops below the positive power supply conductor voltage because of the series connection of the base-to-emitter junctions of the input and feedback transistors therebetween. This voltage drop limits the amplitude of the driving voltage swing which can occur at the input terminal of the current mirror for a given supply. Thus, the foregoing prior art current mirror circuit is not suitable for some low voltage applications.
Other prior art current mirror configurations also suffer from the above problems.